Overvoltage protecting arrangement for an rf amplifier

ABSTRACT

An overvoltage-protecting arrangement comprises a filter connected between an antenna and the input of an RF amplifier for providing impedance matching between the antenna and the amplifier. A diode is connected between the inductor of the filter at a point having an inductance L 1/ omega 2C and a point at ground potential, C being the capacitance between the antenna and ground, omega being 2 pi f and f being the operating frequency of the arrangement. The diode has a zener diode characteristic and is switched to its conductive condition when the antenna supplies a high voltage signal to the filter thereby providing an impedance mismatch between the antenna and the filter.

United States Patent 2,767,309 10/1956 Schaner inventor Michio Kurusu Kawasaki-ski, Japan Appl. No. 685,930 Filed Nov. 27, 1967 Patented June 22, 1971 Assignee Fujitsu Limited Kawasaki, Japan Priority Nov. 29, 1966 Japan 41-78473 OVERVOLTAGE PROTECTING ARRANGEMENT FOR AN RF AMPLIFIER 4 Claims, 15 Drawing Figs.

US. Cl 325/362, 325/379, 325/381, 325/387 Int. Cl. 1104111112, H04b 1/18 FieldolSeareh .f. 325/319, 362, 377, 379, 381, 387; 307/202 References Cited UNITED STATES PATENTS 3,061,785 10/1962 Battin 325/362 3,373,291 3/1968 Peterson 307/202 FOREIGN PATENTS 877,040 9/1961 Great Britain 250/20 Primary Examiner-Robert L. Griffin Assistant Examiner-Kenneth W. Weinstein Attorneys-Curt M. Avery, Arthur E. Wilfond, Herbert L.

Lerner and Daniel J. Tick ABSTRACT: An overvoltage-protecting arrangement cornprises a filter connected between an antenna and the input of an RF amplifier for providing impedance matching between the antenna and the amplifier, A diode is connected between the inductor of the filter at a point having an inductance L=1/w C and a point at ground potential, C being the capacitance between the antenna and ground, to being 21rfand f being the operating frequency of the arrangement. The diode has a uner diode characteristic and is switched to its conductive condition when the antenna supplies a high voltage signal to the filter thereby providing an impedance mismatch between the antenna and the filter.

OVERVOLTAGE PROTECTING ARRANGEMENT FOR AN RF AMPLIFIER DESCRIPTION OF THE INVENTION The present invention relates to an overvoltage-protecting arrangement. More particularly, the invention relates to an overvoltage-protecting arrangement for an RF amplifier.

RF amplifiers in radio receivers and the like are susceptible to damage when a voltage or power of high magnitude is impressed thereon. This may occur, for example, when lightning strikes in the vicinity of the radio equipment or the antenna of such equipment. In such case, the'lightning produces an abnormally large input voltage and power which is considerably above the rated voltage and power of the components of the amplifier and thus either severely damages or completely destroys such components. The RF amplifier of a radio receiver may comprise a transistor, which is destroyed when an input power of tens of watts is applied thereto.

The principal object of the present invention is to provide a new and improved overvoltage-protecting arrangement for an RF amplifier. The overvoltage-protecting arrangement of the present invention protects an RF amplifier from overvoltage. The overvoltage-protecting arrangement of the present invention protects the components of an RF amplifier from overvoltage with efi'iciency, effectiveness and reliability. The overvoltage-protecting arrangement of the present invention is of simple structure. The overvoltage-protecting arrangement of the present invention is economical in manufacture and in operation.

In accordance with the present invention an overvoltageprotecting arrangement for an RF amplifier having an input from an overvoltage in an antenna coupled to the input of the RF amplifier comprises a filter connected between the antenna and the input of the RF amplifier. The filter provides impedance matching between antenna and the RF amplifier and includes an inductor and a capacitor. A diode is connected between the inductor of the filter at a point having an inductance L llw C and a point at ground potential, wherein C is the capacitance between the antenna and ground and m is 2n], f is being the operating frequency of the arrangement. The diode has a Zener diode characteristic and is switched to its conductive condition when the antenna supplies a high voltage signal to the filter thereby providing an impedance mismatch between the antenna and the RF amplifier.

The filter comprises an inductor connected in series between the antenna and the input of the RF amplifier, the inductor having a pair of spaced opposite ends. A first capacitor is connected between one end of the inductor and a point at ground potential and a second capacitor is connected between the other end of the inductor and a point at ground potential. The diode may be connected between an end of the inductor and a point at ground potential. The inductor of the filter has a tap point and the diode may be connected between the tap point of the inductor and a point at ground potential.

The RF amplifier includes an input-coupling transformer having an input winding connected to the inductor of the filter and an output winding inductively coupled to the input winding. The diode may be connected across the output winding of the input-coupling transformer.

' The antenna is connected to a point at ground potential. Another inductor may be connected between an end of the inductor of the filter and a point at ground potential. A resistor may be connected between an end of the inductor of the filter and a point at ground potential. Another diode may be connected in shunt with the first-mentioned diode, the anode of one of said diodes being connected to the cathode of the other of the diodes and the cathode of the one of the diodes being connected to the anode of the other of the diodes. Another inductor may be connected in shunt with the diode.

In order that the present invention may be readily carried into efi'ect, it will now be described with reference to the accompanying drawings, wherein:

FIG. 1 is a circuit diagram of an RF amplifier coupled to an input antenna via a pi-type filter;

FIG. 2 is a circuit diagram of a parallel impedance circuit;

FIG. 3 is a circuit diagram of a series impedance circuit;

FIG 4 is a circuit arrangement for explaining the operation of the overvoltage-protecting arrangement of the present invention;

FIG. 5 is a graphical presentation of a Zener diode characteristic;

FIG. 6 is a circuit diagram illustrating the principle of the present invention;

FIG. 7 is circuit diagram of an embodiment of the overvoltage-protecting arrangement of the present invention;

FIG. 8 is a circuit diagram of a modification of FIG. 7; FIG. 9 is a circuit diagram of another-modification of FIG.

FIG. 10 is a circuit diagram of another embodiment of the overvoltage-protecting arrangement of the present invention;

FIG. 11 is a circuit diagram of a modification of FIG. 10;

FIG. 12 is a circuit diagram of still another embodiment of the overvoltage-protecting arrangement of the present invention;

FIG. 13 is a circuit diagram of a modification of FIG. 12;

FIG. 14 is a circuit diagram of another modification of FIG. 12; and

FIG. 15 is a circuit diagram of still another modification of FIG. 12.

In FIG. I an antenna 11 is coupled to an RF amplifier 12 via a filter 13. The filter 13 is of pi type and comprises an inductor LI connected in series between the antenna 11 and the input of the RF amplifier 12. The inductor LI has a pair of spaced opposite ends 14 and 15. A first capacitor Cl is connected between one end 14 of the inductor L1 and a point at ground potential. A second capacitor C2 is connected between the other end 15 of the inductor L1 and a point at ground potential. The antenna 11 is connected to a point at ground potential.

The RF amplifier 12 may comprise any suitable RF amplifier such as, for example, a transistor 16. An input coupling transformer 17 has an input or primary winding 18 connected to the inductor L1 of the filter 13. The input transformer 17 has an output or secondary winding 19 connected at one end to the base electrode of the transistor 16 and the other end connected to ground via an RC circuit 21. The input and output windings 18 and 19 of the input transformer 17 are inductively coupled to each other. A capacitor 22 is connected in shunt across the input winding 18. The parallel connection of the input winding 18 and the capacitor 22 is connected to a point at ground potential.

The end 15 of the inductor Ll of the filter 13 is connected to a tap point of the input winding 18 of the input transformer 17 of the RF amplifier 12. The emitter electrode of the transistor 16 is connected to ground via an RC circuit 23. The collector electrode of the transistor 16 is connected to one end of an input winding 24 of an output transformer 25. The other end of the input winding 24 of the output transformer 25 is connected to a terminal 26 to which a positive DC bias voltage is applied. A capacitor 27 is connected in shunt across the input winding 24 of the output transformer 25. An output winding 28 of the output transformer 25 is inductively coupled to the input winding 24 of said transformer and provides the amplified output of the RF amplifier l2.

If the impedance ZP of a parallel circuit, as shown in FIG. 2,

comprising a resistor RP and a parallel-corinected inductive reactance XP is converted to its equivalent series impedance 25, comprising a resistor RS and a series-connected inductive fi st filhs fsllea ssssi stisas@22 1: P (RP)XP .RP (XP) Therefore,

If the parallel circuit of FIG. 2 has a quality factor QP, the following equations apply.

In FIG. 4, the circuit comprises an electromotive force 20 and the antenna resistance RP connected in series with each other between circuit points 31 and 32. An inductive reactance LT, which is the inductive reactance XS of FIG. 3, is connected in series with a load resistance RS, which is the same as the series resistance RS of FIG. 3, between the circuit points 31 and 32. A capacitive reactance CT is connected between the circuit point 31 and 32. The maximum transfer efficiency of the electrical power provided by the electromotive force c and applied to the load resistance RS, is provided by interchanging the impedances seen from circuit points 33 and 34.

When the impedances seen at the circuit points 33 and 34 are interchanged and the equations relating to the parallel and series impedances are utilized, the capacitive reactance CT corresponds to the inductive reactance XP of FIG. 2 and the inductive reactance LT corresponds to the inductive reactance XS of FIG. 3. If the quality factor Q! of the parallel circuit of FIG. 2 is then considerably greater than 1, equations (4) and (6) may be combined to provide the equation It may then be assumed that XkXS (7) so that the following equations are true XP=lljwC (8) XSa'wL (9) FIG. is a graphical presentation of the Zener diode characteristic of a diode of which the terminal voltage is maintained at V0, although the voltage exceeds V0 while the current increases. In FIG. 5, the abscissa represents the current I and the ordinate represents the voltage V. The voltage V0 and the current 10, corresponding to such voltage, are indicated by broken lines in FIG. 5.

If a diode having a nonlinear characteristic is utilized as the load resistance RS of FIG. 4, the resistance at the voltage V0 (FIG. 5) is then VO/IO. A suitable diode of this type is a semiconductor diode having a PN junction and operating in the reverse direction, or a silicon diode operating in the forward direction, for example. When the voltage applied to the filter 13 from the antenna 11 exceeds the terminal voltage V0 of FIG. 5, the following equation applies.

10 l'O I'O PT] [T6 "1* 10) 6. Equations I is the current flowing in the load RX, as shown in FIG. 6. Equations (5) and (10) may then be combined to provide the following equation.

RP m 1+QP I (11) Since the resistance RP, the quality factor QP and the voltage V0 of equation (I I) are constant, equation (I 1) indicates that the current I is also constant. Thus, perfect impedance matching is provided by the circuit of FIG. 4 if the current I is constant although the electromotive force e0 increases. However, since the nonlinear diode, which may be utilized, in accordance with the present invention, as the load RX, as explaincd with reference to FIG. 6, has a reverse direction characteristic, the current I is not constant at the voltage V0. This is illustrated by the curve A of FIG. 5. Instead of remaining constant, the current I varies with an increase in terminal voltage VO.

If a diode having a nonlinear characteristic is utilized as the load RS of FIG. 4-and the voltage Vincreases above the magnitude VORS the current I varies and thereby invalidates equation I I). This provides an impedance mismatch at the circuit points 33 and 34 (FIG. 4) so that the voltage and power applied to the load RS is decreased to a considerable extent, since the quality factor QP is considerably greater than I, as hereinbefore indicated.

If the principle of FIG. 4 is utilized in the circuit of FIG. 1, the circuit of FIG. 6 emerges. When the voltage applied to the load RS of FIG. 4 does not exceed V0, the load RX of FIG. 6 has an infinite resistance and may be overlooked. In FIG. 6 the electromotive force e is the input voltage provided by the antenna. The antenna has a resistance RA. The antenna resistance RA is matched with the RF amplifier 12 by a pi-type filter 35. The filter 35 comprises an inductor LT,LA which is connected in series between the antenna and the RF amplifier 12; the antenna being represented by its resistance RA. The inductor LT,LA has a tap point to which one end of the load or diode RX is connected. The other end of the load RX is connected to a point at ground potential. A first capacitor CT is connected between one end of the inductor LT,LA and a point at ground potential. A second capacitor CA is connected between the other end of the inductor LT,LA and a point at ground potential. The tap point of the inductor LT,LA divides said inductor into the inductances or inductive reactances LTand LA.

In FIG. 6, the inductive reactance LT is the same as that of FIG. 4 and the capacitive reactance CT is the same as that of FIG. 4. The inductive reactance LA and the capacitive reactance CA of FIG. 6 are suitably adjusted to provide the same matching between the antenna and the amplifier as the filter 13 of FIG. 1. Thus, the matching characteristics of the pi-type filters l3 and 35 of FIGS. I and 6 respectively, are the same, whereas their quality factors, Q are different. The operation is usually most satisfactory when Q is increased in magnitude. In an ordinary shortwave receiver, which operates at a frequency of 10 to 50 megahertz, a quality factor Q of IO or more may be readily provided. This satisfies the aforementioned relationship wherein QP is considerably greater than 1 and XP is equal to XS(equation 7).

If the power and voltage supplied by the antenna in FIG. 6 is very small, the electromotive force 2, which is also very small, usually has a magnitude of from 0.5 microvolt to 8.0 millivolts. Since the voltage across the load RX of FIG. 6 has a magnitude of less than V0 volts, the impedance of said load becomes infinite and the operation of the circuit of FIG. 6 is the same as that of the circuit of FIG. 1. Such operation is the usual operation of a radio receiver.

When the power and voltage supplied by the antenna in FIG. 6 becomes very large in magnitude, the electromotive force e increases in magnitude and the voltage across the load RX exceeds the magnitude V0. When the voltage across the load RX exceeds the magnitude V0, current 1 flows through said load. Since the load RX has a Zener diode characteristic, as shown by the curve A of FIG. 5, the voltage across said load exceeds the magnitude V0 by a small amount. This prevents damage to or destruction of the components such as, for example, the transistor 16 (FIG. I), of the RF amplifier 12. Furthermore, even if the power and voltage supplied by the antenna are further increased in magnitude, the power consumption of the load RX, which is equal to the product of the voltage V0 and the current I0, increases a small amount. This is due to the provision of an impedance mismatch between the antenna and the RF amplifier 12, as illustrated in FIG. 4, due

to the switching of the diode, which comprises the load RX, to its conductive condition.

The load RX may thus comprise a relatively small resistor. The preferred load RX comprises a silicon Zener diode. A satisfactory load RX comprises an ordinary or usual silicon diode which operates in a low voltage region when it is biased in a forward direction and thereby exhibits a constant voltage or Zener diode characteristic.

In FIG. 7, which illustrates an embodiment of the overvoltage-protecting arrangement of the present invention, the load RX of FIG. 6 comprises a pair of usual silicon diodes DI and D2. In FIG. 7, a filter 36 of pi type couples the antenna II to the RF amplifier I2. The filter 36 comprises an inductor Ll connected in series between the antenna II and the RF ampli fier 12, with one end of said inductor connected to said antenna and the other end of said inductor connected to the input of said amplifier.

The filter 36 also comprises a first capacitor CI connected between one end of the inductor LI and a point at ground potential, and a second capacitor C2 connected between the other end of said inductor and a point at ground potential. An ordinary silicon diode DI is connected between the tap point of the inductor L1 of the filter 36 and a point at ground potential. Another ordinary silicon diode D2 is connected in shunt with the diode D1. The cathode of the diode D1 is connected to the tap point of the inductor LI and the anode of said diode is connected to a point at ground potential. The anode of the diode D2 is connected to the cathode of the diode D1 and the cathode of the diode D2 is connected to the anode of the diode DI.

In the modification of FIG. 8, the load RX of FIG. 6 comprises a diode D3 connected in parallel with an inductor LN. The parallel circuit D3, LN is connected between the tap point of the inductor L1 of the filter 36 and a point at ground potential. In FIG. 8, when the inductor LN has a DC resistance which is very close to zero ohms and an impedance or reactance which'is considerably larger than its inductance, a half-cycle of AC current flows through the inductor Ll of the filter 36.

In the modification of FIG. 9 a diode D4 constitutes the load RX of FIG. 6 and is connected between the tap point of the inductor LI of the filter 36 and a point at ground potential. An inductor LM is connected between one end of the inductor L]! of the filter 36 and a point at ground potential. The modification of FIG. 9 is essentially the same as that of FIG. 8, since the diode D4 is the same as the diode D3 and the inductor LM is the same as the inductor LN. The difference between the modifications of FIG. 9 and FIG. 3 is that the additional inductor LM of FIG. 9 is connected between one end of the inductor LI and ground rather than across the diode D3 and therefore between the tap point of the inductor L1 and ground, as in the modification of FIG. 8. The modification of FIG. 9, in which the additional inductor LM is connected in parallel with the first capacitor CI or the second capacitor C2, is permissible due to the very small DC resistance of the inductor LI.

Since, as indicated in equation (7), XP equals XS when QP is greater than 1, and jwL nearly equals l/jwC,

awn/E 12 and L==llmC (l3) Thus, when the frequency, which is equal to w/Zar, and the capacitance C between the antenna II and ground are known, the inductance L may be determined from equation (13). The tap point of the inductor LI is selected to coincide with the point at which the inductance corresponds to the inductance measured from the antenna side. As in FIG. 9, a diode D4 may be utilized as the load RX of FIG. 6 to provide the overvoltage-protecting arrangement of the present invention. Any suitable type of filter such as, for example, a constant K-type filter, may be utilized as the filter 36, as long as such filter is of an impedance conversion type.

The filter I3 or 36 utilized in the overvoltage-protecting arrangement of the present invention should be a low-pass filter in order to prevent spurious harmonics, so that a pi-type filter, as utilized, is suitable. Furthermore, when the tap point of the inductor LI of the filter 36 approaches the end of said inductor which is connected to the second capacitor C2, said tap may be omitted, and the load RX of FIG. 6 may comprise a diode D5 connected between the end of the inductor LI to which said second capacitor is connected and a point at ground potential, as shown in the embodiment of FIG. 10. In FIG. I0, the diode D5 and the inductor LM are connected in parallel with the second capacitor C2; said diode, said inductor and said second capacitor being connected between the same end of the inductor LI of the filter I3 and a point at ground potential.

In the modification of FIG. II, the inductor LM is replaced by a resistor R. Thus, in FIG. Ill, which is a modification of the embodiment of FIG. III, the diode 5, the resistor R and the second capacitor C2 are connected to the same end of the inductor LI of the filter I3. The resistor R of the modification of FIG. II may, if desired, be connected to the other end of the inductor LI of the filter I3 if such end is at the same DC potential as the end thereof which is connected to the amplifier I2. The resistance value of the resistor R is such that the Rf impedance of the RF amplifier I2 is very slightly affected.

In FIGS. I2, I3, M and I5, the inductor LM is FIG. by the inductance of the usual input-coupling transfomter I7 of the RF amplifier I2. FIGS. I3, M and I5 are different modifications of the embodiment of FIG. I2.

In the embodiment of FIG. 12, the filter 36, which connects the antenna II to the RF amplifier I2, is the same as such filter of the modification of FIG. 9. The difference between the modification of FIG. 9 and the embodiment of FIG. 12 is that the inductor LM of FIG. 9 is eliminated in FIG. I2. In FIG. I2, the load RX of FIG. 6 comprises the diode D -I (FIG. 9) connected between the tap point of the inductor LI of the filter 36 and a point at ground potential.

The modification of FIG. I3 is the same as that of FIG. II, with the exception that the resistor R of FIG. II is eliminated in FIG. I3. Thus, in the modification of FIG. I3, the filter I3 and the diode D5 are the same as those of the modification of FIG. II.

The modification of FIG. I41 is the same as the embodiment of FIG. 12 with the exception of a resistor R which is connected to the same end of the inductor LI of the filter 36 as the second capacitor C2. In the modification of FIG. I4, the diode D6 is connected between the tap point of the inductor L1 and a point at ground potential.

The modification of FIG. 15 is the same as that of FIG. 13, except that the diode D5, which constitutes the load of the modification of FIG. I3, is eliminated in FIG. I5. In the modification of FIG. IS, the load RX of FIG. 6 comprises a diode D6 which is connected across the secondary or output winding 19 of the input-coupling transformer I7 of the RF amplifier I2. The diode D6 of the modification of FIG. I5 is thus protected by the input transformer I7, which functions to prevent frequencies which are considerably different from the signal frequency from reaching said diode.

An advantage of the modification of FIG. I5 is that interrnodulation crosstalk caused by the nonlinearity of the diode D6 is reduced relative to such crosstalk in a circuit wherein such diode is connected to the primary or input winding of the input transformer I7. The receiver which includes the RF amplifier I2, receives, via the modification of FIG. I5, signals of from 0.5 microvolt to about 3 millivolts and the overvoltageprotecting arrangement transfers such signals to the RF amplifier I2 without loss, as in a conventional receiver. When a large input voltage and power is supplied by the antenna II, the voltage applied to the diode D6 changes the impedance of said diode to such an extent that it produces an impedance mismatch between said antenna and the RF amplifier 12. Consequently, the increased voltage and power are reflected and their transmission to the amplifier is prevented by the limiting action of the diode D6. The components of the amplifier are thus protected from overvoltage.

The diode D1, D3, D4, D or D6 may comprise any suitable diode such as, for example, a known type of silicon diode, a diode having a low rating, a diode having a Zener diode characteristic, or a Zener diode.

While the invention has been described by means of specific examples and in specific embodiments, I do not wish to be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

lclaim:

1. An overvoltage-protecting arrangement for an RF amplifier having an input from an overvoltage in an antenna coupled to the input of said RF amplifier, said RF amplifier including an input-coupling transformer having an input winding and an output winding inductively coupled to said input winding said overvoltage-protecting arrangement comprising filter means connected between said antenna and the input of said RF amplifier for providing impedance matching between said antenna and said RF amplifier, said filter means comprising an inductor connected in series between said antenna and the input winding of the inputcoupling transformer of said RF amplifier, said inductor having a pair of spaced opposite ends, a first capacitor connected between one end of said inductor and a point at ground potential and a second capacitor connected between the other end of said inductor and a point at ground potential; and

a diode connected between the inductor of said filter means at a portion of the inductor having an inductance L=l/ (MC and a point at ground potential and across the output winding of the input-coupling transformer of said RF amplifier, wherein C is the capacitance between said antenna and ground and m is 21r f,fbeing the operating frequency of said arrangement, said diode having a Zener diode characteristic and being switched to its conductive condition when said antenna supplies a high voltage signal to said filter means thereby providing an impedance mismatch between said antenna and said RF amplifier.

2. An overvoltage-protecting arrangement for an RF amplifier having an input from an overvoltage in an antenna coupled to the input of said RF amplifier, said overvoltage-protecting arrangement comprising filter means connected between said antenna and the input of said RF amplifier for providing impedance matching between said antenna and said RF amplifier, said filter means comprising an inductor connected in series between said antenna and the input of said RF amplifier, said inductor having a pair of spaced opposite ends, a first capacitor connected between one end of said inductor and a point at ground potential and a second capacitor connected between the other end of said inductor and a point at ground potential;

a diode connected between an end of the inductor of said filter means at a portion of the inductor having an inductance L=1/ wC and a point at ground potential, wherein C is the capacitance between said antenna and ground and w is 21rf,f being the operating frequency of said arrangement, said diode having a Zener diode characteristic and being switched to its conductive condition when said antenna supplies a high voltage signal to said filter means thereby providing an impedance mismatch between said antenna and said RF amplifier; and 5 another inductor connected between an end of the inductor of said filter means and a point at ground potential.

3. An overvoltage-protecting arrangement for an RF amplifier having an input from an overvoltage in an antenna coupled to the input of said RF amplifier, said overvoltage-protecting arrangement comprising filter means connected between said antenna and the input of said RF amplifier for providing jmpedance matching between said antenna and sald R amplifier, said filter means comprising an inductor connected in series between said antenna and the input of said RF amplifier, said inductor having a pair of spaced opposite ends, a first capacitor connected between one end of said inductor and a point at ground potential and a second capacitor connected between the other end of said inductor and a point at ground potential;

a diode connected between an end of the inductor of said filter means at a portion of the inductor having an inductance L=l/wC and a point at ground potential, wherein C is the capacitance between said antenna and ground and w is 21rf, f being the operating frequency of said arrangement, said diode having a Zener diode characteristic and being switched to its conductive condition when said antenna supplies a high voltage signal to said filter means thereby providing an impedance mismatch between said antenna and said RF amplifier; and

a resistor connected between an end of the inductor of said filter means and a point at ground potential.

4. An overvoltage-protecting arrangement for an RF amplifier having an input from an overvoltage in an antenna coupled to the input of said RF amplifier, said overvoltage-protecting arrangement comprising filter means connected between said antenna and the input of said RF amplifier for providing impedance matching between said antenna and said RF amplifier, said filter means comprising an inductor connected in series between said antenna and the input of said RF amplifier, said inductor having a pair of spaced opposite ends, a first capacitor connected between one end of said inductor and a point at ground potential and a second capacitor connected between the other end of said inductor and a point at ground potential, said inductor having a tap point;

a diode connected between the tap point of the inductor of said filter means at a portion of the inductor having an inductance [Fl/0 C and a point at ground potential, wherein C is the capacitance between said antenna and ground and w is 21rf,f being the operating frequency of said arrangement, said diode having a Zener diode characteristic and being switched to its conductive condition when said antenna supplies a high voltage signal to said filter means thereby providing an impedance mismatch between said antenna and said RF amplifier; and

another inductor connected in shunt with said diode. 

1. An overvoltage-protecting arrangement for an RF amplifier having an input from an overvoltage in an antenna coupled to the input of said RF amplifier, said RF amplifier including an inputcoupling transformer having an input winding and an output winding inductively coupled to said input winding said overvoltage-protecting arrangement comprising filter means connected between said antenna and the input of said RF amplifier for providing impedance matching between said antenna and said RF amplifier, said filter means comprising an inductor connected in series between said antenna and the input winding of the input-coupling transformer of said RF amplifier, said inductor having a pair of spaced opposite ends, a first capacitor connected between one end of said inductor and a point at ground potential and a second capacitor connected between the other end of said inductor and a point at ground potential; and a diode connected between the inductor of said filter means at a portion of the inductor having an inductance L 1/ omega 2C and a point at ground potential and across the output winding of the input-coupling transformer of said RF amplifier, wherein C is the capacitance between said antenna and ground and omega is 2 pi f, f being the operating frequency of said arrangement, said diode having a Zener diode characteristic and being switched to its conductive condition when said antenna supplies a high voltage signal to said filter means thereby providing an impedance mismatch between said antenna and said RF amplifier.
 2. An overvoltage-protecting arrangement for an RF amplifier having an input from an overvoltage in an antenna coupled to the input of said RF amplifier, said overvoltage-protecting arrangement comprising filter means connected between said antenna and the input of said RF amplifier for providing impedance matching between said antenna and said RF amplifier, said filter means comprising an inductor connected in series between said antenna and the input of said RF amplifier, said inductor having a pair of spaced opposite ends, a first capacitor connected between one end of said inductor and a point at ground potential and a second capacitor connected between the other end of said inductor and a point at ground potential; a diode connected between an end of the inductor of said filter means at a portion of the inductor having an inductance L 1/ omega 2C and a point at ground potential, wherein C is the capacitance between said antenna and ground and omega is 2 pi f, f being the operating frequency of said arrangement, said diode having a Zener diode characteristic and being switched to its conductive condition when said antenna supplies a high voltage signal to said filter means thereby providing an impedance mismatch between said antenna and said RF amplifier; and another inductor connected between an end of the inductor of said filter means and a point at ground potential.
 3. An overvoltage-protecting arrangement for an RF amplifier having an input from an overvoltage in an antenna coupled to the input of said RF amplifier, said overvoltage-protecting arrangement comprising filter means connected between said antenna and the input of said RF amplifier for providing impedance matching between said antenna and said RF amplifier, said filter means comprising an inductor connected in series between said antenna and the input of said RF amplifier, said inductor having a pair of spaced opposite ends, a first capacitor connected between one end of said inductor and a point at ground potential and a second capacitor connected between the other end of said inductor and a point at ground potential; a diode connected between an end of the inductor of said filter means at a portion of the inductor having an inductance L 1/ omega 2C and a point at ground potential, wherein C is the capacitance between said antenna and ground and omega is 2 pi f, f being the operating frequency of said arrangement, said diode having a Zener diode characteristic and being switched to its conductive condition when said antenna supplies a high voltage signal to said filter means thereby providing an impedance mismatch between said antenna and said RF amplifier; and a resistor connected between an end of the inductor of said filter means and a point at ground potential.
 4. An overvoltage-protecting arrangement for an RF amplifier having an input from an overvoltage in an antenna coupled to the input of said RF amplifier, said overvoltage-protecting arrangement comprising filter means connected between said antenna and the input of said RF amplifier for providing impedance matching between said antenna and said RF amplifier, said filter means comprising an inductor connected in series between said antenna and the input of said RF amplifier, said inductor having a pair of spaced opposite ends, a first capacitor connected between one end of said inductor and a point at ground potential and a second capacitor connected between the other end of said inductor and a point at ground potential, said inductor having a tap point; a diode connected between the tap point of the inductor of said filter means at a portion of the inductor having an inductance L 1/ omega 2C and a point at ground potential, wherein C is the capacitance between said antenna and grOund and omega is 2 pi f, f being the operating frequency of said arrangement, said diode having a Zener diode characteristic and being switched to its conductive condition when said antenna supplies a high voltage signal to said filter means thereby providing an impedance mismatch between said antenna and said RF amplifier; and another inductor connected in shunt with said diode. 